Run Elasticsearch-dependent tests also in CI

**Is your feature request related to a problem? Please describe.**

We have a few deactivated tests for Elasticsearch which currently need to be run manually.

**Describe the solution you'd like**

There is a simple solution to have an ES container spawned just for testing, using this in the build gradle file:

```
testImplementation "org.testcontainers:testcontainers:1.20.0"
testImplementation "org.testcontainers:junit-jupiter:1.20.0"
testImplementation "org.testcontainers:elasticsearch:1.20.0"
```

But: It has to be tested if this actually works in the GitHub CI.

**Describe alternatives you've considered**

Boring scripts, lame mocking, ...

**Additional context**

-

Start by locating the deactivated Elasticsearch tests and the build Gradle file, then verify the proposed Testcontainers dependencies in GitHub CI. Done means those tests run automatically in CI against an Elasticsearch container without manual setup.

Written by the indexing model from the issue text.